Lady in waiting: Samantha Cameron

The woman who aspires to be the next prime ministerial wife may not be overtly political but she is already playing a powerful role

(DAN KITWOOD / GETTY IMAGES)

Slowly, unavoidably, Samantha Cameron is becoming a public figure. The cameras were trained on her mercilessly this week as she was put through the ordeal of hearing her husband speak about the death of their young child, in a hall packed with strangers.

"I know what sustains me the most," David Cameron added. "She is sitting right there, and I'm incredibly proud to call her my wife." When he had finished, he went into the audience to bring his wife on stage for the kiss that decorated the front pages.

The Conservative leader does not shy from using his happy family for political effect, to the extent even of inviting cameras into the kitchen of the home, and posing for publicity photographs with his children, including Ivan, who died in February aged six, his short life blighted by cerebral palsy.

Though "Sam Cam" has never spoken publicly about her husband's career, nor taken centre stage like Cherie Blair or Sarah Brown, she has been less protective of her young children's privacy than either of those women. And whether she wants it or not, she must by now have steeled herself for the years of relentless exposure that lie ahead.

Yet, curiously, this is a woman who, until she met David Cameron, did not display the least interest in politics. She was a rather wild art student in Bristol, trying to shake off an aristocratic upbringing by getting a dolphin tattoo on her ankle, and hanging out with the likes of the musician and actor Adrian Thaws, known as Tricky, who was heavily into drugs.

In 1992, when she was 21, her friend Clare Cameron invited her to join a family holiday in Tuscany, where she began a romance with Clare's 26-year-old brother. He was a more serious person than her bohemian friends, a figure to be reckoned with in Westminster, despite his youth. He was political adviser to the Chancellor, Norman Lamont, but that cut no ice among Samantha's Bristol friends, who found the idea that anyone would belong to the Conservative Party rather ridiculous.

One of the first stories told about Samantha in the biography of David Cameron, by Francis Elliott and James Hanning, is of the telephone ringing when they were trying to enjoy a quiet weekend in Bristol. Samantha called out from the bed: "If that's Norman Lamont, tell him to fuck off."

Several years later, Bruce Anderson, a commentator for this newspaper, discovered that she did not know the meaning of the expression "One-Nation" Conservatism, which dates back to Benjamin Disraeli, and refers to the socially conscious left wing of the party. This small gap in her knowledge did not bother Samantha Cameron. "I may not have read history at Oxbridge, but I know as much about politics as most people and it means nothing to me," she said.

In that reply lies a clue as to why Cameron and others around him hold her in such esteem. Most of the Cameron circle dived into politics after university, and have limited experience of anything else. Cameron was a researcher at Conservative Central Office at the age of 21. But all professional politicians pay homage to the importance of knowing "real people", and in their world, Samantha Cameron is a real person, an outsider with a real job. She works four days a week as creative director of Smythson, the upmarket stationers which boasts of being one of the oldest shops in New Bond Street.

She once had an ambition to be a professional painter, but that did not work out, and she started working in 1996 as a professional window dresser and interior designer. She had done holiday work in Smythson years earlier, having been eased in there by her mother's social connections, and was given a commission to redesign the shop, which led to her becoming its creative director at the age of only 25.

David Cameron's commitment to education and health reforms is said to be driven by his wife. As the mother of two surviving children, Nancy and Arthur, aged six and four, and particularly as Ivan's mother, she has been a constant consumer of public services like no one else in the Cameron circle, mentally comparing their performance to the way that a private business like Smythson has to care for its customers if it is to survive.

But the notion that she is a busy Mrs Everywoman, juggling work and family commitments, is somewhat misleading. Her family background, for a start, is so extraordinary that it is like something out of a novel by Nancy Mitford. Her father, Sir Reginald Sheffield, is an authentic Tory toff, with a family pedigree going back several centuries. He put in an appearance at this week's Conservative conference, as president of the Brigg and Goole Conservative Association, having arrived late because one of his dogs had to be taken to a vet, and was overheard in a crowded reception talking on a mobile phone, probably to his daughter, saying loudly: "No, I'm not getting in any trouble – no champagne."

Michael Brown, who was his local MP for many years, said: "He's an absolutely delightful character, from the old Tory school. He enjoys horse riding and shooting. He's not a sharp dresser, lots of Wellington boots and mud. People recognise him because he invariably wears a bow tie. This is old money, frayed at the edges, all inherited furniture."

Samantha's mother and maternal grandmother were even more extraordinary. Her grandmother, Pandora Clifford, an interior designer, scandalously became pregnant by an older man, a one-legged war veteran, when she was 17. He did the decent thing and married her, but she later left him to marry Viscount Astor.

Her daughter, Annabel, conceived outside marriage, is managing director of the upmarket mail-order furniture business Oka. She was 21 when she married Sir Reginald, but they were divorced in the 1970s, and she married William Astor, her stepfather's nephew, who inherited his title in 1972. Viscount Astor was a minister in John Major's government for five years.

Despite the mazurka of Sheffield/ Astor divorces and remarriages, Samantha Sheffield's childhood was comfortable and secure. Her mother and fathers remained such good friends that, after her father remarried, she effectively had two sets of parents, seven siblings or half-siblings, and a number of very desirable properties she could call home.

Though her father is nowhere near as wealthy as her stepfather, he is by most people's reckoning a very rich man, richer by far than David Cameron's multimillionaire dad. When she once described herself as having grown up "near Scunthorpe", she was referring to the 3,000 acres of arable land in north Lincolnshire that her father inherited, along with Thealby Hall. The main family seat, Normanby Hall, was handed over to Lincolnshire council in lieu of death duties, but there is still Sutton Park, a £5m mansion near York with another 1,000 acres, which her father inherited in 1997.

Her family's connections have been useful to her husband. When he lost his job in government after the Black Wednesday fiasco in 1992, Lady Astor had a word with her friend Michael Green, chairman of Carlton Television, who hired Cameron as director of corporate affairs, the only job he has ever held outside politics. Her money also helped to buy their first home, in North Kensington, in 1992, for £215,000. It is now valued at £1.5m.

As a director of Smythson, she is likely to be paid more than her husband, whose salary as Leader of the Opposition is £130,000 a year. In one interview, David Cameron also mentioned that his wife "owns a field in Scunthorpe". That may not sound like much, but her father has been heard to say: "I live off unearned income, garnished by the occasional planning consent." Were his daughter's "field" to be similarly garnished, it could be very valuable indeed.

But for all her inherited wealth, Samantha Cameron is no idle daughter of the aristocracy. She is a shrewd and confident woman, liked by most people who meet her, who knew she was marrying a wannabe MP and has decided to back him all the way.

There was a time when a prime minister's wife could stay out of the public eye, if she wanted. Audrey Callaghan managed it. Norma Major tried. But even if Samantha Cameron suddenly turns publicity-shy after the next election, it will be too late. She has already had too much exposure to slip out of sight.

A life in brief

Born: 18 April 1971, London. Her parents are Sir Reginald Adrian Berkeley Sheffield, 8th Baronet, a descendant of King Charles II, and Annabel Lucy Veronica Astor.

Educated: St Helen and St Katharine School in Abingdon, Oxfordshire; took A-levels at Marlborough College in Wiltshire. Completed an art foundation course at Camberwell College of Arts and went on to study fine art at the University of the West of England, Bristol

Family: Married David Cameron in 1996. They have two children, Nancy Gwen (born 2004) and Arthur Elwen (born 2006). The couple's disabled son Ivan (born 2002) died earlier this year.

Career: She has worked for the luxury goods retailer Smythson since 1996, becoming its creative director in 2006. This year she won the Glamour magazine award for best accessory designer. She co-owns Oka, an interior design and lighting shop in west London with her mother.

She says: "I'm a working mum. We have to pay for childcare. I don't have huge amounts of spare cash to spend on designer clothing. I think all working mums are a bit like that."

They say: She will always say what she thinks regardless of who's sitting there. She comes from a long line of strong women who were in the right place when the right genes were being handed out." Jane Churchill, designer and friend